1 /*! 2 @file 3 Defines `boost::hana::detail::unpack_flatten`. 4 5 @copyright Louis Dionne 2013-2017 6 Distributed under the Boost Software License, Version 1.0. 7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) 8 */ 9 10 #ifndef BOOST_HANA_DETAIL_UNPACK_FLATTEN_HPP 11 #define BOOST_HANA_DETAIL_UNPACK_FLATTEN_HPP 12 13 #include <boost/hana/at.hpp> 14 #include <boost/hana/config.hpp> 15 #include <boost/hana/detail/algorithm.hpp> 16 #include <boost/hana/detail/array.hpp> 17 #include <boost/hana/length.hpp> 18 #include <boost/hana/unpack.hpp> 19 20 #include <cstddef> 21 #include <utility> 22 23 24 BOOST_HANA_NAMESPACE_BEGIN namespace detail { 25 template <std::size_t ...Lengths> 26 struct flatten_indices { 27 // avoid empty arrays by appending 0 to `lengths` 28 static constexpr std::size_t lengths[sizeof...(Lengths) + 1] = {Lengths..., 0}; 29 static constexpr auto flat_length = 30 detail::accumulate(lengths, lengths + sizeof...(Lengths), 0); 31 32 template <bool Inner> computedetail::flatten_indices33 static constexpr auto compute() { 34 detail::array<std::size_t, flat_length> indices{}; 35 for (std::size_t index = 0, i = 0; i < sizeof...(Lengths); ++i) 36 for (std::size_t j = 0; j < lengths[i]; ++j, ++index) 37 indices[index] = (Inner ? i : j); 38 return indices; 39 } 40 41 static constexpr auto inner = compute<true>(); 42 static constexpr auto outer = compute<false>(); 43 44 template <typename Xs, typename F, std::size_t ...i> 45 static constexpr decltype(auto) applydetail::flatten_indices46 apply(Xs&& xs, F&& f, std::index_sequence<i...>) { 47 return static_cast<F&&>(f)( 48 hana::at_c<outer[i]>(hana::at_c<inner[i]>( 49 static_cast<Xs&&>(xs) 50 ))... 51 ); 52 } 53 }; 54 55 struct make_flatten_indices { 56 template <typename ...Xs> 57 auto operator()(Xs const& ...xs) const -> detail::flatten_indices< 58 decltype(hana::length(xs))::value... 59 >; 60 }; 61 62 template <typename Xs, typename F> unpack_flatten(Xs && xs,F && f)63 constexpr decltype(auto) unpack_flatten(Xs&& xs, F&& f) { 64 using Indices = decltype(hana::unpack(xs, make_flatten_indices{})); 65 return Indices::apply(static_cast<Xs&&>(xs), static_cast<F&&>(f), 66 std::make_index_sequence<Indices::flat_length>{}); 67 } 68 } BOOST_HANA_NAMESPACE_END 69 70 #endif // !BOOST_HANA_DETAIL_UNPACK_FLATTEN_HPP 71